The present invention relates to the regulation of a clamp assembly in a injection molding machine and, more particularly, to a closed loop system for controlling the pressure of a clamp assembly to optimize production efficiency in an injection molding machine.
Injection molding machines are used to mass produce plastic parts by automated injection of plastic into a mold that is opened and closed by a clamp assembly. At the end of an injection molding cycle, the clamp assembly is retracted, and the part is ejected. The next cycle is initiated by motion of the clamp from its retracted position to a position where the mold is nearly closed. During the cycle the mold machine should not be subjected to excessive pressure and forces generated by parts left behind in the mold, or incorrect positioning of the mold. In the event that parts are left behind nor incorrectly positioned in the mold the machine must be stopped and the piece removed. This causes production delay which impacts production efficiency.
More particularly, a typical injection molding process utilizes a injection molding machine 10, illustrated in FIG. 1, wherein plastic pellets (not shown) are melted and forced into a mold 20, by a clamp assembly 113 that opens and closes mold 20 during the mold injection cycle.
The clamp assembly 113 comprises a hydraulic cylinder which is mechanically coupled to a piston 27. Clamp assembly 113 further comprises a stationary platen 13 which is mechanically affixed to a mold 20, and a moving platen 12, which is coupled to piston 27. Piston 27 is adapted to hydraulically traverse from a clamp closed position to a clamp open position in an injection molding cycle by variable force applied to piston 27. A hydraulic cylinder 11 is movably coupled to piston 27. The variable force applied to piston 27 by the fluid in hydraulic cylinder 11 may be controlled by a control unit 28.
The injection molding process comprises four successive stages. The first stage called xe2x80x9cplasticationxe2x80x9d comprises steps wherein the plastic pellets are pushed forward from a hopper 17 through a barrel 21 towards a nozzle 14 by a rotating screw 22 while being heated in barrel 21 by electric heater bands 23 surrounding barrel 21. The second stage called xe2x80x9cinjection,xe2x80x9d occurs when the plastic is pushed through nozzle 14 into a mold 20 by clamp assembly 113. The third stage called xe2x80x9cpacking,xe2x80x9d occurs when mold 20 is packed with the molten plastic. The fourth stage called xe2x80x9ccooling,xe2x80x9d occurs when mold 20 is cooled to solidify the plastic part therein. After the completion of the solidification and cooling stages, piston 27 is retracted and the part is ejected. The injection molding machine 10 is thus ready for the process to be repeated in the next cycle.
It is desirable to protect the injection mold machine 10 during motion of piston 27 by xe2x80x9csmoothingxe2x80x9d the response of the clamp assembly cycle during clamp closure of piston 27 so as to prevent damage to the machine from plastic parts, remaining in the mold after having been jammed in mold 20 during the previous cycle. It is also desirable to control the stroke of piston 27 during the injection molding cycle so as to minimize the duration of the clamp cycle.
The present invention provides an apparatus and method for controlling the force and velocity of a piston within the clamp assembly of a injection molding machine. The control unit comprises the following elements: a cascade compensator, wherein the cascade compensator is adapted to generate a force error signal having lead and lag compensation based on the desired force minus the actual force of the piston; an estimator coupled to the clamp assembly, wherein the estimator is utilized to estimate the position of the servo valve and to generate a corresponding actual force signal; a velocity and position controller coupled to the clamp assembly and the estimator, wherein the velocity and position controller is adapted to generate a velocity error signal based on the piston velocity signal; and an auto selector coupled to the cascade compensator, wherein the auto selector is adapted to select between the velocity error signal and the force error signal so as to generate the valve position command signal.
The present invention additionally employs a method for determining the velocity profile of the piston of a clamp assembly, wherein the velocity profile having a ramp-up interval, a constant speed interval, ramp-down interval; and a final touchdown interval. The method comprises the following steps: determining the maximum displacement xf of the servo valve; selecting a final touch down velocity vs of the piston; choosing a ratio xcex1 of the piston ramp-down interval to the servo valve displacement xe2x80x9cxvxe2x80x9d during the ramp-up interval, selecting a desired maximum piston velocity (vmax) during the ramp-up interval and constant speed interval; and determining servo valve displacement xu from simulations or estimate from previous cycles.